VDAC
Voltage-dependent anion channel (VDAC; mitochondrial porin) is a pore-forming protein found in the outer mitochondrial membrane in all eukaryotic cells controlling the fluxes of ions and metabolites between the mitochondria and the cytosol. VDAC is recognized as a key protein in mitochondria-mediated apoptosis due to its function in the release of apoptotic proteins located in the inter-membranal space and its interaction with apoptotic proteins. VDAC also serves as binding sites for several cytosolic enzymes and mitochondrial intermembranal space proteins, including hexokinase, creatine kinase and glycerol kinase.
Three mammalian isoforms of VDAC are known, VDAC1, VDAC2, VDAC3, where VDAC1 is the major isoform expressed in mammalian cells. Blachly-Dysion et al (1993) disclosed the cloning and functional expression in yeast of two human VDAC isoforms, VDAC1 and VDAC2. Human VDAC1 was shown to specifically bind hexokinase. U.S. Pat. No. 5,780,235 discloses two novel VDAC sequences, which were named HACH (Human voltage-dependent Anion CHannel), subsequently identified as VDAC2 and VDAC3. That patent provides genetically engineered expression vectors, host cells containing the vector, a method for producing HACH and a method for identifying pharmaceutical compositions inhibiting the expression and activity of HACH and for the use of such compositions for the treatment of cancer and proliferative diseases.
Apoptosis
Apoptosis, also known as programmed cell death, plays a central role in, inter alia, development, immune cell regulation and tissue homeostasis in multicellular organisms. Genetic and molecular analysis from various species has indicated that the apoptotic pathway is highly conserved. In addition to being essential for normal development and maintenance, apoptosis is important in the defense against viral infection and in preventing cancer.
Mitochondria play an important role in the regulation of apoptotic cell death. The release of apoptogenic intermediates such as cytochrome c from the intermembranal space into the cytoplasm of a cell initiates a cascade of caspase activation that executes the cell death program. Substantial evidence links VDAC to apoptosis and suggests that VDAC is a critical player in the release of apoptogenic proteins from mitochondria in mammalian cells (Shoshan-Barmatz and Gincel, 2003).
Anti-Apoptotic and Pro-Apoptotic Proteins
Diverse intrinsic cell death signals emanating from various subcellular organelles can induce the release of cytochrome c from mitochondria. The Bcl-2 family of pro- and anti-apoptotic proteins constitutes a decisive control point for apoptosis. Proteins in the Bcl-2 family are major regulators of apoptosis (reviewed in Kim, 2005). Members of this family include both pro- and anti-apoptotic proteins and share homology in up to four conserved regions termed Bcl-2 homology (BH) 1-4 domains. The family can be divided into three main sub-classes: anti-apoptotic proteins, pro-apoptotic proteins and BH3-only proteins.
The anti-apoptotic proteins, which include hexokinase-I (HK-I), Bcl-2 and Bcl-xL, share homology throughout all four BH domains. The pro-apoptotic proteins can be further subdivided and include multidomain proteins, such as Bax and Bak, which possess sequence homology in BH1-3 domains.
The more distantly related BH3-only proteins appear to be only pro-apoptotic and share sequence homology within the BH3 region, which is required for their apoptotic function. The BH3-only proteins include, for example, BID, NOXA, PUMA and BAD.
It is currently held that anti-apoptotic members of the Bcl-2 family of proteins, such as HK-I, HK-II, Bcl-2 and Bcl-xL, act to promote cell survival by interacting with VDAC. Conversely, pro-apoptotic members of the Bcl-2 family of proteins, including Bak and Bax, may interact with VDAC to promote release of cytochrome c. Because of the pivotal role that mitochondria play in apoptotic cell death, mitochondrial proteins serve as potential targets for apoptosis regulating therapies.
One major obstacle in cancer chemotherapy is inherent, or acquired, resistance, apparently due to the suppression of apoptosis. Hexokinase-I (HK-I) is an anti-apoptotic mitochondrial protein that binds to VDAC. Many tumor cells exhibit a high glycolytic rate, which is correlated with a high level of HK-I expression. It is believed that the overexpression of anti-apoptotic proteins such as HK-I in cancer cells is a self-defense mechanism of those cells and is related to the cell's resistance to chemotherapy. It would be useful to develop agents that overcome apoptosis suppression in cancer cells, including HK-I suppression.
Certain compositions related to VDAC and use thereof for either inhibiting or inducing apoptosis are known in the art. US Patent Application Publication No. 2005/0085420 discloses methods of inhibiting apoptosis by promoting formation of a BAK/VDAC2 complex, and methods of promoting apoptosis by disrupting formation of a BAK/VDAC2 complex. The VDAC2/BAK inhibitor compound is, for example, a BH3 domain peptide, a BH3 domain-only mutein, an anti-VDAC2 antibody, a VDAC2 mutein and the like.
US Patent Application Publication No. 2005/0234116 discloses small molecule compounds with utility as VDAC regulators, in particular as apoptosis suppressors.
There remains an unmet need for therapeutic agents effective in regulating apoptosis and specifically in inducing apoptosis in hyperproliferative disease and inhibiting apoptosis in neurodegenerative, cardiac and ophthalmic diseases. The art neither teaches nor suggests inducing apoptosis by expression of exogenous VDAC1 or the modulation of VDAC1 interaction with its associated proteins using VDAC1 derived peptides.